Problem with this fizzy sound

[sickman82]

Haha.. Steve, I think you might be right there. 'Cork sniffer', I haven't heard that in a long time. I didn't notice whether or not my volume pot sounded scratchy.. either way, if it's good enough for the old Vox and Marshalls I have looked at today, it's good enough for me.

I'm getting to the point now where I just think enough is enough. I lifted the film caps, no difference there whatsoever. I don't want to make any permanent alterations to the PCB in hope that it might fix a problem, that if I'm completely honest, I only ever notice when I'm playing at home and have never noticed at rehearsal or while playing live. I can use my TS808 at home for mild overdrive.

If there were a concrete solution to the issue, without having to spend ridiculous amounts of money by sending it off to Juleamps, or Ben Fargen. Or, if there was a mod that was tried and tested by many Night Train owners, which solved the issue.. I would do it. But, for now.. I think I'll leave it as it is. I have got at least some of what I wanted, an increase in low end. Whether the mod to the input section helps with pedals remains to be seen, but I'm happy leaving it like that for the time being.

Silvio, I'll stay involved in this thread. If you continue with your mods, good luck and keep posting on here with results etc. I'll be interested to see where you get to with it.

[Steve Conner]

Well, one easy mod you could try is, get a 12AT7 (ECC81) tube and swap it out. Try both preamp positions, though I suspect V4 might give the greatest effect with the master volume turned down.

Using the wrong tube randomises things a bit, and might tip the tone in a direction that you prefer. If you don't like it, it is easily reversible.

[Silvio55]

Iīm back after a couple days away! I got the stuff to star modding now, but I donīt think Iīll have time today. Sickman, yes, my night train has that copper strip that always intrigued me, cause it was just conected to one leg of R23 and nowhere else. Itīs sad that your "informant" doesnīt told you more about what to do to fix the problem, or at least what you could try. Iīm posting this days if I have some news!

[pdf64]

'changes to the preamp, power supply and the output section. I add some capacitance to the Plate supply and I add inductance as well'
Regards the above mods, I doubt they would address the 'fizzyness with MV low' issue - any changes to the power supply or power amp only tend to affect high power operation.
As suggested, probably best to leave the MV up and use a pedal.
Pete.

[diagrammatiks]

man a cathodyne with a post master.

genius! a fizzy genius.

the fizziness is gonna be a problem with the cathodyne overdriving way more then any cathodyne was ever intended to be overdriven.

there's no fix to your problem.

that circuit is just poorly designed for what you want to do.

[Chuck H]

Easy thee captain sunshine... You'll make someone cry.

I do tend to agree though. With the master up the power tubes clipping would help mask the problem. Maybe one reason you don't hear this in 5e3 type amps. Another could be the bias conditions for the cathodyne. The 5e3 and NT have different voltages on the cathodyne but the same circuit values. This would make for different bias conditions. Has anyone experimented with the bias resistor on the cathodyne yet? How about that enormous cap feeding the PI. Has anyone tried reducing that to 2200pf yet?

[Silvio55]

Hi Chuck! I still donīt have 1/2 watt resistors for the cathodyne bias (maybe some 1/4 but I have to check), Iīll see if can get them today, do you suggest to try higher or lower values for the bias resistor? I have 2n2 and 4n7 caps and will see if they make any difference.

[sickman82]

As a side note, does anyone know where I can source a suitable choke for this amp in the UK? That's one mod that I am still interested in doing. I really hope you can get to the bottom of the fizziness Silvio, you are a lone crusader! Good luck man. Let us know how you get on, but I have a feeling that Chuck and diagrammatiks are right.

[jmaf]

As a side note, does anyone know where I can source a suitable choke for this amp in the UK?

choke fender | eBay

[Silvio55]

Put a 1M resistor in V4 pin 2 and replaced the gridstoppers in the power tubes with 100k and no difference at all, nothing.
I was looking at the copper strip and yes itīs a shielding, it covers the trace that goes from C19 and R20 to R22, the trace goes side by side with a trace that links 3 big caps; C11, C12 and C13 (3300 uf 10V) from the heater supply, there are no leads that goes near that trace, so I guess the shielding has something to do with those caps...



Hereīs an image of an early PCB without the copper shielding, the red lines are where the shielding is located in later PCBīs.

[diagrammatiks]

you need to stop throwing money at this problem.

as you've already figured out just changing a few values isn't going to make any difference at all.

that p.i. stage needs a complete redesign to get rid of the fizziness problem...

or you just need to turn up your master.

[sickman82]

choke fender | eBay

There are none in the UK on eBay, but thanks anyway.

Any one of you UK members know where supplies chokes? I've checked around a few of the places that I would expect to see them being sold, but they don't sell them. Is there something I'm missing here? Can you get chokes in the UK?

you need to stop throwing money at this problem.

as you've already figured out just changing a few values isn't going to make any difference at all.

that p.i. stage needs a complete redesign to get rid of the fizziness problem...

or you just need to turn up your master.

Money isn't the issue for me, it's the knowledge that to get rid of the problem (presuming that's even possible) I'll have to alter the amp too much. That isn't what I'm interested in. I'm happy to use a pedal with the MV down, as that gets rid of the issue when I'm playing at home. When I'm rehearsing or gigging, the MV is cranked and I don't even think about the fizzyness.

Have you tried the bias resistor option yet Silvio? What results did you get with the coupling cap swap? Keep it coming!

[pdf64]

See Hammond transformer range at Bluebell Untitled Document
Phil is a good guy.
I don't perceive a benefit in using a choke compared to the 1k etc resistor in this application, ie EL84 PP cathode bias amps; rather a disadvantage in that the tube screens are run much harder, due to the lower volt drop across the choke. Generally have to use much higher value screen grid resistors to limit dissipation, which causes local negative feedback and may be perceived to be detrimental to the tone.
I look to get around 500 ohms resistance between the plate and screen nodes, and use a series resistor to achieve that as necessary.
From that, a good choice may be the 154G (9H, 40mA, 700ohms).
Pete.

[Silvio55]

Ok, I tried Changing C10 with 2n2 but nothing again. This is turning a bit frustrating but I wonīt give up now! Maybe itīs true that it isnīt fixable with just changing a few values but I think itīs worth to keep on trying, I have nothing to loose. Iīll try to get 2k2 and 1k resistors for the cathodyne bias, when I get them Iīll report.

[pdf64]

Consider also getting a 1M audio pot that you could 'lash up' between the 2 V4 stages.
A rough and ready implementation could quickly confirm whether the cathodyne overdrive character was the problem.